Cholesteryl-containing ionic liquid crystals composed of alkylimidazolium cations and different anions

Article abstract of DOI:10.1016/j.molstruc.2014.07.026

Cholesteryl-containing ionic liquid crystals (ILCs) 1-cholesteryloxycarbonylmethyl(propyl)-3-methyl(butyl)imidazolium chlorides ([Ca-Me-Im]Cl, [Ca-Bu-Im]Cl, [Cb-Me-Im]Cl and [Cb-Bu-Im]Cl) and corresponding imidazolium tetrachloroaluminates ([Ca-Me-Im]AlCl₄, [Ca-Bu-Im]AlCl₄, [Cb-Me-Im]AlCl₄ and [Cb-Bu-Im]AlCl₄) were synthesized in this work, and the chemical structure, LC behavior and ionic conductivity of all these ILCs were characterized by several technical methods. The imidazolium-based salts with Cl^- ions showed chiral smectic A (S_A^?) phase on both heating and cooling cycles, while the tetrachloroaluminates exhibited chiral nematic (N^?) phase. The mesophase was confirmed by characteristic LC textures observed by polarizing optical microscopy and typical diffractogram obtained by X-ray diffraction measurements. The samples with similar cholesteryl-linkage component showed similar phase transition temperature and entropy, indicating the cholesteryl component influence predominately on the phase transition rather than alkyl substituents on the imidazole ring. The imidazolium tetrachloroaluminates display relatively low phase transition temperature compared with the precursor chlorides. The functional difference in LC behavior and ionic conductivity were discussed by investigated the structural difference between the Cl^--containing and AlCl₄ - containing materials. The imidazolium chlorides exhibited layer structure both in crystal and mesophase states, and should be organized with a 'head-to-tail' organization to form interdigitated monolayer structures due to the tight ion pairs. But the imidazolium tetrachloroaluminates displayed layer structure only in crystal phase, and should be organized in 'head-to-head' arrangements form bilayer structures due to loose combination of ion pairs despite of hydrogen-bond and electrostatic attraction interaction.

Full text of DOI:10.1016/j.molstruc.2014.07.026

Journal of Molecular Structure 1075 (2014) 515–524
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Cholesteryl-containing ionic liquid crystals composed of
alkylimidazolium cations and different anions
⇑
Xin Lan, Lu Bai, Xin Li, Shuang Ma, Xiaozhi He, Fanbao Meng
The Research Centre for Molecular Science and Engineering, Northeastern University, Shenyang 110004, PR China
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
ꢀ
Imidazolium-based ionic liquid crystals with cholesteryl mesogens are prepared.
The ionic liquid crystals include chlorides and tetrachloroaluminates.
The imidazolium chlorides showed chiral smectic A phase.
The imidazolium tetrachloroaluminates exhibit chiral nematic phase.
a r t i c l e i n f o
a b s t r a c t
Article history:
Cholesteryl-containing ionic liquid crystals (ILCs) 1-cholesteryloxycarbonylmethyl(propyl)-3-
methyl(butyl)imidazolium chlorides ([Ca-Me-Im]Cl, [Ca-Bu-Im]Cl, [Cb-Me-Im]Cl and [Cb-Bu-Im]Cl) and
Received 16 March 2014
Received in revised form 9 July 2014
Accepted 9 July 2014
corresponding imidazolium tetrachloroaluminates ([Ca-Me-Im]AlCl
and [Cb-Bu-Im]AlCl ) were synthesized in this work, and the chemical structure, LC behavior and ionic
conductivity of all these ILCs were characterized by several technical methods. The imidazolium-based
4 4 4
, [Ca-Bu-Im]AlCl , [Cb-Me-Im]AlCl
4
Available online 16 July 2014
À
*
A
salts with Cl ions showed chiral smectic A (S ) phase on both heating and cooling cycles, while the tet-
Keywords:
Liquid crystals
Ionic
Chiral
Phase behavior
*
rachloroaluminates exhibited chiral nematic (N ) phase. The mesophase was confirmed by characteristic
LC textures observed by polarizing optical microscopy and typical diffractogram obtained by X-ray dif-
fraction measurements. The samples with similar cholesteryl-linkage component showed similar phase
transition temperature and entropy, indicating the cholesteryl component influence predominately on
the phase transition rather than alkyl substituents on the imidazole ring. The imidazolium tetrachloroa-
luminates display relatively low phase transition temperature compared with the precursor chlorides.
The functional difference in LC behavior and ionic conductivity were discussed by investigated the struc-
À
tural difference between the Cl -containing and AlCl
4
—containing materials. The imidazolium chlorides
exhibited layer structure both in crystal and mesophase states, and should be organized with a ‘head-
to-tail’ organization to form interdigitated monolayer structures due to the tight ion pairs. But the
imidazolium tetrachloroaluminates displayed layer structure only in crystal phase, and should be
organized in ‘head-to-head’ arrangements form bilayer structures due to loose combination of ion pairs
despite of hydrogen-bond and electrostatic attraction interaction.
Ó 2014 Published by Elsevier B.V.
Introduction
imperfection (e.g. high melting points) has recently emerged for
chiral LCs.
Liquid crystals (LCs) are usually regarded as the fourth state of
matter between anisotropic solid and isotropic liquid, and are one
kind of soft materials for applications with a long time. Chiral LCs
can exhibit a marvelous variety of LC phases including cholesteric
phase, chiral smectic phase, and blue phases, and have presented
large potential for various applications [1–4]. However, some
Ionic liquids (ILs) possess a lot of unique properties such as
wide liquid temperature range, low volatility, negligible vapor
pressure, high thermal stability, good solubility in a wide range
of materials, high ionic conductivity, and nonflammability [5–7].
Thus ILs have attracted considerable interest as soft materials for
various applications including solvents for chemical synthesis,
catalyst, ionic conduction and so on over the past decade [8–10].
Ionic liquid crystals (ILCs) are one kind of ILs exhibiting liquid
crystalline properties, which are composed of ionic species and
have anisotropic structural organizations in a certain temperature
⇑
Corresponding author.
E-mail address: mengfb@mail.neu.edu.cn (F. Meng).
http://dx.doi.org/10.1016/j.molstruc.2014.07.026
022-2860/Ó 2014 Published by Elsevier B.V.
0

5
16
X. Lan et al. / Journal of Molecular Structure 1075 (2014) 515–524
range. Both from fundamental and from applied points of view,
ILCs are a fascinating class of functional soft materials that com-
bine the properties of ILs and LCs, and have attracted considerable
attention recently [11–13]. In fact, ILs can be appropriately
designed to obtain liquid crystalline. For example, many imidazo-
lium-based ILCs can be generated simply by increasing the alkyl
chain length to increase the self-organization character [14,15].
The structure of ILs goes through a transition from spatially
heterogeneous to ILCs due to the increased van der Waals interac-
tions between the increased alkyl length of cationic side-chains,
which is observed by Ji et al. using coarse-grained molecular
dynamics simulations [16]. For ILCs, the liquid-crystalline behavior
strongly depends on the nature of both the cation and the anion,
but also on the length of the alkyl chain of the organic cation
(Varian Associates, Palo Alto, CA) with fourier transform with
dimethyl sulfoxide-d (DMSO-d ) or CDCl as a solvent and tetra-
6
6
3
methylsilane (TMS) as an internal standard. The element analyses
(EA) were carried out by using an Elementar Vario EL III (Elemen-
tar, Germany). Differential scanning calorimetry (DSC) analyses
were carried out on a NETZSCH instruments DSC 204 (Netzsch,
À1
Wittelsbacherstr, Germany) at a heating rate of 10 °C min under
nitrogen atmosphere. Polarizing optical microscopy (POM) was
carried out with a Leica DMRX (Leica, Wetzlar, Germany) micro-
scope coupled with a Linkam Scientific Instrument THMSE-600
hot stage and a TMS94 temperature controller (Linkam, Surrey,
England). X-ray diffraction (XRD) studies were carried out on a
Cu Ka (k = 1.542 Å) radiation monochromatized with a Rigaku
DMAX-3A X-ray diffractometer (Rigaku, Japan). Ionic conductivi-
ties were measured with a RTS-9 four-probe meter (Guangzhou
four-point probe technology, Guangzou, China) equipped with a
hot stage and temperature controller.
[
17]. Many types of ILCs have been investigated, which contain dif-
ferent organic cations such as imidazolium [18–20], pyridinium
21–23], pyrrolidinium [24,25], quaternary ammonium and qua-
ternary phosphonium salts [26–28] conjugating different anions
[
À
À
À
3
such as halide ion, BF
4
, PF
6
, CF
3
SO
and so on. Most reported ILCs
General procedure for the synthesis of cholesteryl chloro-
alkylcarboxylic esters
contain single organic cation unit in one ILC molecule. Recently,
ILCs containing multiple imidazolium units are reported by Schen-
kel, which can exhibit smectic A phase [29]; and viologen-based
ILCs with bipyridinium cores are reported by Casella, which exhibit
rich polymorphism including the ionic smectic A phase and an
ordered smectic phase [30]. In particular, imidazolium-based ionic
liquids attract considerable attention among these types of ILCs
because they give low melting points with high chemical and elec-
trochemical stability, and exhibit excellent ability in anisotropic
ionic conductivity, organized reaction media or ordered solvents
To a solution of cholesterol (0.05 mol, 19.3 g) and pyridine
(20 mL) in tetrahydrofuran (THF, 100 mL) was added dropwise
with 0.06 mol of 2-chloroacetyl chloride (or 4-chlorobutanoyl
chloride), and the reaction mixture was refluxed and stirred for
48 h. Then the mixture was cooled to room temperature, poured
into cold water (100 mL) and acidified with 6 N sulfuric acid. The
precipitates were isolated by filtration and dried in a vaccum oven,
and white crystals of crude product were obtained by recrystalliza-
tion of the precipitates in chloroform.
[
31,32]. Usually, the most common long alkyl-substituted imidazo-
lium-based ILCs only show smectic A phases [33]. However, some
imidazolium sulphonates ILCs display cubic phases and hexagonal
columnar phases [34]. Thus it is interesting to investigate the
liquid-crystalline behaviors of imidazolium-based ILCs that con-
tain some functional groups.
Cholesteryl 2-chloroacetate (Ca-Cl)
Yield 87%. IR (KBr): 2941, 2867, 1726, 1468, 1375, 1196, 686.
Elem. analysis: found: 75.33% C, 10.15% H; calc.: 75.21% C,
1
10.23% H. H NMR (CDCl
3
, d, ppm): 0.71–2.01 (m, 41H), 2.38 (t,
In this work, we synthesized a series of imidazolium-based ILCs
containing cholesteryl mesogenic groups. Goossens et al. have
reported the first imidazolium-based ILCs containing cholesteryl
substituents which are connected with a flexible undecyl spacer
to the cationic core [35]. Their compounds incorporate a longer
and more flexible alkyl spacer between the cholesteryl mesogens
and the imidazolium ion pairs, exhibiting chiral smectic A phases
at room temperature. In comparison to these cholesteryl-contain-
ing ILCs, it is interesting to study the liquid-crystalline behaviors
of these kinds of ILCs containing short alkyl spacers between the
cholesteryl mesogens and the imidazolium ion pairs. A series of
cholesteryl-containing imidazolium-based ILCs with different
J = 6 Hz, 2H), 3.78–3.84 (m, 2H), 4.66–4.73 (m, 1H), 5.41 (t,
J = 6 Hz, 1H). C NMR (CDCl , d, ppm): 12.9, 18.3, 21.1, 23.3,
3
23.5, 24.5, 24.6, 24.8, 28.0, 28.1, 31.1, 35.9, 36.0, 36.1, 37.4, 37.5,
38.7, 39.1, 39.2, 41.5, 41.7, 46.2, 50.3, 53.5, 56.3, 74.7, 121.8,
140.1, 172.8.
1
3
Cholesteryl 4-chlorobutanoate (Cb-Cl)
Yield 83%. IR (KBr): 2947, 2863, 2835, 1722, 1465, 1190, 683.
Elem. analysis: found: 75.53% C, 10.34% H; calc.: 75.80% C,
1
10.47% H. H NMR (CDCl
3
, d, ppm): 0.64–2.02 (m, 43H), 2.36 (t,
J = 6 Hz, 2H), 2.75–2.79 (m, 2H), 3.76–3.81 (m, 2H), 4.65–4.71 (m,
1
3
3
1H), 5.39 (t, J = 7 Hz, 1H). C NMR (CDCl , d, ppm): 12.8, 18.4,
À
À
4
short chain lengths and anions of Cl , and AlCl
were synthesized
19.1, 21.2, 23.4, 23.6, 24.7, 24.8, 24.9, 28.1, 28.2, 28.3, 31.3, 35.9,
36.0, 36.1, 36.2, 37.4, 37.5, 38.8, 39.0, 39.1, 41.6, 41.7, 46.3, 50.2,
53.6, 56.2, 56.5, 74.8, 121.9, 140.2, 172.4.
in this work. The effect of the different substituent was studied by
comparing the structures, thermal behaviors, and cation–anion
interactions of the cholesteryl-containing imidazolium salts.
Synthesis of imidazolium chloride salts
Experimental
1-Methylimidazole (0.06 mol, freshly distilled over KOH) was
added to a solution of 0.05 mol cholesteryl chloro-alkylcarboxylic
ester (Ca-Cl or Cb-Cl) in dry acetonitrile (50 mL) under a nitrogen
atmosphere. The solution was refluxed and stirred for 72 h. The
solvent was removed under reduced pressure, and the crude prod-
uct was washed three times with diethyl ether, dried under high
vacuum for 24 h to obtain methylimidazolium chloride salts. The
product was then transferred into a glovebox for storage.
Materials and methods
Cholesterol, 1-methylimidazole, 1-butylimidazole, aluminum
trichloride, 2-chloroacetyl chloride and 4-chlorobutanoyl chloride
were obtained from ALDRICH. All commercially available chemi-
cals were used without further purification except 1-methylimid-
azole and 1-butylimidazole. FT-IR spectra of the synthesized
materials were obtained by the KBr method performed on Perkin-
Elmer instruments Spectrum One Spectrometer (PerkinElmer,
1-Cholesteryloxycarbonylmethyl-3-methylimidazolium chloride
([Ca-Me-Im]Cl)
Yield 95%. IR (KBr): 2936, 2867, 1723, 1466, 1376, 1182. Elem.
analysis: found: 72.53% C, 9.73% H, 5.18% N; calc.: 72.69% C,
1
13
Foster City, CA). H NMR (300 MHz) and C NMR (75 MHz) spectra
were obtained with a Varian Gemini 300 NMR Spectrometer

X. Lan et al. / Journal of Molecular Structure 1075 (2014) 515–524
517
1
1H). 13C NMR (DMSO-d
24.4, 24.6, 24.7, 24.8, 24.9, 27.8, 28.2, 28.3, 32.5, 32.7, 36.4, 36.5,
37.1, 37.2, 37.3, 37.5, 39.6, 41.5, 41.8, 46.4, 50.5, 54.4, 56.5, 56.8,
73.9, 122.8, 122.9, 124.9, 138.6, 140.6, 173.2.
9
2
1
9
2
3
5
.80% H, 5.14% N. H NMR (DMSO-d
6
, d, ppm): 0.68–2.01 (m, 41H),
6
, d, ppm): 12.8, 18.8, 19.1, 19.4, 21.4,
.27 (t, J = 6 Hz, 2H), 3.81 (s, 3H), 4.20–4.24 (m, 2H), 4.45-4.59 (m,
H), 5.38 (t, J = 6 Hz, 1H), 7.73 (d, J = 6 Hz, 1H), 7.76 (d, J = 6 Hz, 1H),
.11 (s, 1H). C NMR (DMSO-d , d, ppm): 12.7, 18.7, 19.1, 19.3,
6
1.3, 24.5, 24.6, 24.7, 24.8, 24.9, 27.9, 28.0, 28.1, 32.6, 32.7, 36.4,
6.5, 37.1, 37.3, 37.4, 37.6, 39.5, 41.3, 41.4, 46.3, 50.5, 54.7, 56.6,
6.9, 73.8, 122.7, 122.9, 124.4, 138.2, 140.7, 173.0.
1
3
1
-Cholesteryloxycarbonylpropyl-3-methylimidazolium
tetrachloroaluminate ([Cb-Me-Im]AlCl
IR (KBr): 3066, 2937, 2867, 2421, 1723, 1641, 1565, 1467, 1376,
4
)
1
-Cholesteryloxycarbonylpropyl-3-methylimidazolium chloride
1
1
(
4
269, 1190. H NMR (DMSO-d
t, J = 5 Hz, 2H), 2.44–2.55 (m, 2H), 3.83 (s, 3H), 4.18–4.21 (m, 2H),
.44–4.58 (m, 1H), 5.39 (t, J = 6 Hz, 1H), 7.75 (d, J = 5 Hz, 1H), 7.81
6
, d, ppm): 0.64–1.98 (m, 43H), 2.25
(
[Cb-Me-Im]Cl)
Yield 93%. IR (KBr): 3062, 2941, 2865, 2833, 2421, 1724, 1643,
1
4
553, 1467, 1191. Elem. analysis: found: 73.12% C, 10.01% H,
13
(
6
d, J = 5 Hz, 1H), 9.19 (s, 1H). C NMR (DMSO-d , d, ppm): 12.8,
.98% N; calc.: 73.33% C, 10.02% H, 4.89% N. 1H NMR (DMSO-d
6
,
1
2
4
1
8.2, 19.0, 19.1, 19.2, 21.3, 24.5, 24.6, 24.7, 24.8, 24.9, 28.1, 28.2,
8.3, 32.7, 32.8, 36.2, 36.3, 37.5, 37.6, 37.8, 37.9, 39.9, 41.7, 41.8,
6.5, 50.3, 54.7, 56.7, 56.8, 73.9, 122.0, 122.8, 122.9, 138.9, 140.3,
73.0.
d, ppm): 0.64–1.98 (m, 43H), 2.26 (t, J = 7 Hz, 2H), 2.45–2.56 (m,
H), 3.83 (s, 3H), 4.18–4.21 (m, 2H), 4.44–4.58 (m, 1H), 5.39 (t,
J = 7 Hz, 1H), 7.75 (d, J = 6 Hz, 1H), 7.81 (d, J = 6 Hz, 1H), 9.19 (s,
2
1
3
1
2
3
5
6
H). C NMR (DMSO-d , d, ppm): 12.9, 18.2, 19.0, 19.1, 19.2,
1.3, 24.5, 24.6, 24.7, 24.8, 24.9, 28.0, 28.1, 28.2, 32.7, 32.9, 36.3,
6.4, 37.5, 37.6, 37.8, 37.9, 39.8, 41.8, 41.9, 46.4, 50.3, 54.8, 56.8,
6.9, 73.9, 121.9, 122.7, 122.8, 138.8, 140.2, 173.1.
The butylimidazolium chloride salts were synthesized by the
same method using 1-butylimidazole and cholesteryl chloro-alkyl-
carboxylic esters.
1-Cholesteryloxycarbonylmethyl-3-butylimidazolium
tetrachloroaluminate ([Ca-Bu-Im]AlCl
IR (KBr): 3072, 2948, 2407, 1722, 1639, 1581, 1467, 1312, 1195.
H NMR (DMSO-d , d, ppm): 0.64–2.00 (m, 48H), 2.26 (t, J = 6 Hz,
H), 4.07–4.13 (m, 2H), 4.18–4.21 (m, 2H), 4.44-4.55 (m, 1H),
.39 (t, J = 6 Hz, 1H), 7.74 (d, J = 6 Hz, 1H), 7.77 (d, J = 6 Hz, 1H),
4
)
1
6
2
5
9
1
3
5
1
3
6
.16 (s, 1H). C NMR (DMSO-d , d, ppm): 12.8, 18.3, 19.1, 19.2,
1
-Cholesteryloxycarbonylmethyl-3-butylimidazolium chloride
9.3, 21.3, 24.7, 24.6, 24.7, 24.8, 24.9, 27.9, 28.0, 28.1, 32.8, 32.9,
6.2, 36.4, 37.6, 37.7, 37.8, 37.9, 39.9, 41.8, 42.6, 46.4, 50.8, 54.6,
6.6, 56.8, 73.9, 121.9, 122.7, 122.8, 138.8, 140.3, 173.1.
(
[Ca-Bu-Im]Cl)
Yield 91%. IR (KBr): 2936, 2867, 2833, 1723, 1466, 1376, 1213,
1
7
1
(
1
188. Elem. analysis: found: 73.46% C, 10.02% H, 4.68% N; calc.:
3.62% C, 10.13% H, 4.77% N. 1H NMR (DMSO-d
, d, ppm): 0.65–
.99 (m, 48H), 2.25 (t, J = 6 Hz, 2H), 4.06–4.12 (m, 2H), 4.19–4.22
6
1-Cholesteryloxycarbonylpropyl-3-butylimidazolium
tetrachloroaluminate ([Cb-Bu-Im]AlCl
IR (KBr): 3075, 2947, 2867, 2418, 1723, 1637, 1467, 1218, 1189.
H NMR (DMSO-d , d, ppm): 0.65–2.04 (m, 50H), 2.27 (t, J = 6 Hz,
H), 2.44–2.60 (m, 2H), 4.07–4.14 (m, 2H), 4.19–4.27 (m, 2H),
.44–4.57 (m, 1H), 5.38 (t, J = 6 Hz, 1H), 7.75 (d, J = 6 Hz, 1H),
4
)
m, 2H), 4.43–4.55 (m, 1H), 5.39 (t, J = 6 Hz, 1H), 7.76 (d, J = 6 Hz,
H), 7.80 (d, J = 6 Hz, 1H), 9.18 (s, 1H). 1 C NMR (DMSO-d
3
6
, d,
1
6
ppm): 12.7, 18.3, 19.1, 19.2, 19.3, 21.3, 24.5, 24.6, 24.7, 24.8,
2
4
7
1
2
4
1
2
3
1
4.9, 27.9, 28.0, 28.1, 32.8, 32.9, 36.2, 36.4, 37.6, 37.7, 37.8, 37.9,
9.9, 41.9, 42.0, 46.5, 50.4, 54.7, 56.6, 56.8, 73.8, 122.0, 122.8,
22.9, 138.9, 140.3, 173.3.
1
3
6
.79 (d, J = 6 Hz, 1H), 9.18 (s, 1H). C NMR (DMSO-d , d, ppm):
2.9, 18.6, 19.2, 19.2, 19.4, 21.9, 24.4, 24.5, 24.6, 24.7, 24.9, 28.1,
8.2, 28.4, 32.6, 32.8, 36.2, 36.4, 37.6, 37.7, 37.9, 38.0, 39.5, 41.5,
1.7, 46.7, 50.8, 54.5, 56.6, 56.9, 73.9, 121.9, 122.7, 122.9, 138.8,
40.5, 173.2.
1
-Cholesteryloxycarbonylpropyl-3-butylimidazolium chloride
(
[Cb-Bu-Im]Cl)
Yield 91%. IR (KBr): 2936, 2867, 1723, 1466, 1375, 1213, 1188.
Elem. analysis: found: 74.21% C, 10.21% H, 4.38% N; calc.: 74.17%
C, 10.32% H, 4.55% N. 1H NMR (DMSO-d
, d, ppm): 0.63–2.08 (m,
0H), 2.26 (t, J = 6 Hz, 2H), 2.47–2.61 (m, 2H), 4.03–4.11 (m, 2H),
.18–4.23 (m, 2H), 4.46–4.56 (m, 1H), 5.39 (t, J = 6 Hz, 1H), 7.73
6
Results and discussion
5
4
Syntheses
1
3
(
(
2
3
1
d, J = 6 Hz, 1H), 7.78 (d, J = 6 Hz, 1H), 9.16 (s, 1H). C NMR
DMSO-d , d, ppm): 12.6, 18.3, 19.2, 19.2, 19.3, 21.8, 24.4, 24.5,
4.6, 24.7, 24.8, 28.1, 28.2, 28.3, 32.4, 32.8, 36.2, 36.3, 37.6, 37.7,
7.8, 37.9, 39.6, 41.6, 41.8, 46.8, 50.7, 54.6, 56.7, 56.9, 73.8,
21.8, 122.6, 122.8, 138.9, 140.6, 173.6.
6
The synthesis of the target chlorine-containing cholesteric
liquid crystals, the imidazolium chlorides and the imidazolium tet-
rachloroaluminates are outlined in Fig. 1. The cholesteryl chloro-
alkylcarboxylic esters (Ca-Cl and Cb-Cl) were synthesized through
one step esterization reaction by use of cholesterol and chloro-
alkyl acyl chlorides in THF and pyridine. The imidazolium chloride
salts [Ca-Me-Im]Cl, [Ca-Bu-Im]Cl, [Cb-Me-Im]Cl and [Cb-Bu-Im]Cl
were prepared from N-alkyl imidazole and alkyl chlorides accord-
ing to the general procedure reported by previous literatures
[36,37]. The imidazolium tetrachloroaluminates were obtained
via one step reaction among the imidazolium chloride salts and
Synthesis of imidazolium tetrachloroaluminates
To 0.05 mol of the obtained methyl(butyl)imidazolium chlo-
rides, dry aluminum trichloride (0.05 mol, 6.65 g) was added in
small portions. The flask was cooled to 0 °C during the addition
due to the exothermic reaction while forming the imidazolium tet-
rachloroaluminates. No further purification was attempted, and
3 3
AlCl in a 1:1 mol ratio. The purification of AlCl and the prepara-
the product was then transferred into a N
for storage.
2
-atmosphere glovebox
tion of the imidazolium tetrachloroaluminates were described
earlier [38,39]. The chemical structure characterization of the syn-
thesized materials is in good agreement with the prediction, which
is performed by EA, FT-IR spectra, H and C NMR spectra.
The NMR spectra of chlorine-containing LC Cb-Cl, the corre-
sponding N-methylimidazolium chloride salt [Cb-Me-Im]Cl, and
1
13
1
-Cholesteryloxycarbonylmethyl-3-methylimidazolium
tetrachloroaluminate ([Ca-Me-Im]AlCl
IR (KBr): 3087, 2946, 2419, 1721, 1638, 1567, 1467, 1230, 1189.
H NMR (DMSO-d , d, ppm): 0.67–2.00 (m, 41H), 2.26 (t, J = 6 Hz,
H), 3.82 (s, 3H), 4.21–4.25 (m, 2H), 4.44–4.58 (m, 1H), 5.39 (t,
J = 6 Hz, 1H), 7.75 (d, J = 6 Hz, 1H), 7.78 (d, J = 6 Hz, 1H), 9.13 (s,
4
)
1
6
4
tetrachloroaluminate salt [Cb-Me-Im]AlCl are displayed in Fig. 2.
1
2
In H NMR spectra, as can be observed in Fig. 2a and c for com-
pounds of Cb-Cl and [Cb-Me-Im]Cl, the formation of imidazolium

5
18
X. Lan et al. / Journal of Molecular Structure 1075 (2014) 515–524
Fig. 1. Synthesis route of the chlorine-containing cholesteric liquid crystals, the imidazolium chlorides and the imidazolium tetrachloroaluminates.
salts has an effect on the chemical shift of some hydrogens includ-
ing near-oxygen hydrogens (AOCHA in cholesteryl) and near-ole-
characteristic bands of imidazole ring in the chemical structure,
and the peaks at 3067 and 2421 cm could be assigned to forma-
À1
fin hydrogens (AOCCH
of imidazolyl hydrogens. For the imidazolium tetrachloroalumi-
nate salt [Cb-Me-Im]AlCl , all hydrogens did not display any shifts
2
C@CA in cholesteryl) besides appearance
tion of hydrogen bonds [40–42]. While in the spectrum of imidazo-
lium chloride salt [Cb-Me-Im]Cl, the peak intensity of 3067 and
2421 cm was much weaker than those of imidazolium tetrachlo-
À1
4
in comparison with the imidazolium chloride salt [Cb-Me-Im]Cl, as
shown in Fig. 2c and e. It is important to note that the same behav-
ior is observed for cholesteryl chloro-alkylcarboxylic ester Ca-Cl;
other imidazolium chloride salts [Ca-Me-Im]Cl, [Ca-Bu-Im]Cl, and
roaluminate. This result indicates that effect of hydrogen bonding
in imidazolium tetrachloroaluminates should be much stronger
than in imidazolium chloride salts due to more chlorine component
in the tetrachloroaluminate systems.
[
Cb-Bu-Im]Cl; and other imidazolium tetrachloroaluminates [Ca-
Me-Im]AlCl , [Ca-Bu-Im]AlCl , and [Cb-Bu-Im]AlCl
4
4
4
. In 13C NMR
Mesomorphic properties of cholesteryl chloro-alkylcarboxylic esters
specta, as can be observed in Fig. 2b and d for chlorine-containing
compound Cb-Cl and corresponding N-methylimidazolium chlo-
ride salt [Cb-Me-Im]Cl, some new carbon chemical shift around
The synthesized cholesteryl chloro-alkylcarboxylic esters Ca-Cl
and Cb-Cl displayed enantiotropic cholesteric mesophases on
heating and cooling cycles by investigating with DSC and POM.
The DSC thermograms and some characteristic optical textures of
Ca-Cl and Cb-Cl are exhibited in Fig. 4. The DSC curves of Ca-Cl
1
23 and 139 ppm appeared due to carbons on imidazole units in
the chemical structure, and other carbons did not show significant
shifts. For the imidazolium tetrachloroaluminate [Cb-Me-Im]AlCl
all carbons did not display significant shifts in comparison with
4
,
showed a crystalline melting endotherm at 163.2 °C (enthalpy
À1
[
Cb-Me-Im]Cl, as shown in Fig. 2d and f.
changes d H = 47.13 kJ mol
)
and cholesteric-isotropic phase
À1
The chemical structure of chlorine-containing cholesteric LCs
Ca-Cl and Cb-Cl), imidazolium chloride salts, and imidazolium tet-
transition at 196.8 °C (0.67 kJ mol ) on heating, as well as an
À1
(
isotropic-cholesteric transition at 177.2 °C (À1.32 kJ mol ) and
À1
rachloroaluminates was also confirmed by FT-IR spectra analysis.
The FT-IR spectra of Cb-Cl, N-methyl imidazolium chloride salt
crystallization at 132.2 °C (À17.26 kJ mol ) on cooling, as dis-
played in Fig. 4a. In Fig. 4b, Cb-Cl exhibited a crystalline melting
À1
[
Cb-Me-Im]Cl and corresponding imidazolium tetrachloroalumi-
endotherm at 80.0 °C (d H = 39.45 kJ mol ) and cholesteric-isotro-
pic phase transition at 122.3 °C (3.27 kJ mol ) on heating, as well
À1
nate salt [Cb-Me-Im]AlCl
teristic bands at 2947–2835, 1722, 1190, and 683 cm in Fig. 3a
4
is shown in Fig. 3. Cb-Cl showed charac-
À1
À1
as an isotropic-cholesteric transition at 116.3 °C (À4.33 kJ mol
)
À1
attributable to alkyl CAH stretching, ester C@O, CAO stretching in
and crystallization at 75.3 °C (À12.84 kJ mol ) on cooling.
ester and CACl stretching bands respectively. For [Cb-Me-Im]AlCl
4
,
Observed by POM, Ca-Cl and Cb-Cl exhibited cholesteric tex-
tures including oily streak and focal conic texture upon heating
and cooling circles, as illustrated in Fig. 4. Oily streak texture
(see Fig. 4c and e) is one of the most commonly observed textures
of the chiral nematic phase prepared between two untreated glass
the CACl stretching could hardly be observed, but some new mid-
dle-strength infrared absorption peaks appeared at 3067, 2421,
À1
1
641, 1555 cm in Fig. 3c suggesting formation of imidazolium
À1
tetrachloroaluminate. The peaks at 1641 and 1555 cm
were

X. Lan et al. / Journal of Molecular Structure 1075 (2014) 515–524
519
Fig. 2. NMR spectra: (a) 1H NMR spectrum for cholesteryl 4-chlorobutanoate (Cb-Cl); (b) C NMR spectrum for Cb-Cl; (c) 1H NMR spectrum for 1-cholesteryloxycarbon-
13
1
3
1
ylpropyl-3-methylimidazolium chloride ([Cb-Me-Im]Cl); (d)
C
NMR spectrum for [Cb-Me-Im]Cl; (e)
H NMR spectrum for 1-cholesteryloxycarbonylpropyl-3-
1
3
methylimidazolium tetrachloroaluminate ([Cb-Me-Im]AlCl ); (f) C NMR spectrum for [Cb-Me-Im]AlCl .
4
4
that is often observed in cholesterics is fan-shaped or fan-like tex-
ture. Fan-like textures are observed for strongly twist materials,
while fan-shaped textures that exhibit focal conics (see Fig. 4d
and f) are found for materials with a slightly smaller twist [37].
Liquid-crystalline behavior of imidazolium chloride salts
The liquid-crystalline behavior of all the synthesized imidazoli-
um chloride salts [Ca-Me-Im]Cl, [Ca-Bu-Im]Cl, [Cb-Me-Im]Cl and
[
Cb-Bu-Im]Cl were investigated with DSC, POM, and XRD. Their
phase transition temperatures and corresponding enthalpy
changes are summarized in Table 1. For [Ca-Me-Im]Cl and [Ca-
Bu-Im]Cl which are derived from the same cholesteryl chloro-
alkylcarboxylic ester Ca-Cl, they showed similar phase transition
temperature and entropy, as well [Cb-Me-Im]Cl and [Cb-Bu-Im]Cl.
It indicates that different carbon chains of substituent group on
imidazole ring influence little on the phase transition. While for
Fig. 3. FTIR spectra for (a) cholesteryl 4-chlorobutanoate (Cb-Cl), (b) 1-choleste-
ryloxycarbonylpropyl-3-methylimidazolium chloride ([Cb-Me-Im]Cl) and
c) 1-cholesteryloxycarbonylpropyl-3-methylimidazolium tetrachloroaluminate
[Cb-Me-Im]AlCl ).
[
Ca-Me-Im]Cl and Cb-Me-Im]Cl which contain the same N-methyl
(
(
imidazole component and different colesteryl components, the
former displayed lower melting point, as well [Ca-Bu-Im]Cl and
4
[
Cb-Bu-Im]Cl.
substrates. These oily streaks can be seen as a network of defect
lines dispersed in uniformly helical regions. Another nature texture
All these imidazolium chloride salts showed similar liquid-crys-
talline behavior as list in Table 1, thus the mesomorphic properties

5
20
X. Lan et al. / Journal of Molecular Structure 1075 (2014) 515–524
respectively; and an isotropic-to-S*
phase transition and crystalli-
A
zation appeared at 149.0 °C and 121.7 °C respectively during cool-
ing scans, as shown in Fig. 5a. Observed by POM, [Ca-Me-Im]Cl
*
exhibited an enantiotropic S
A
texture upon heating and cooling.
When [Ca-Me-Im]Cl was heated to 127 °C, the sample began to
melt, and mesomorphic properties appeared; a fan-shaped texture
*
of the S
A
phase gradually appeared, as displayed in Fig. 5b. The
director basically lay in the plane of the substrate and the smectic
layers were curved across the fans in this fan-shaped texture, and it
*
was the most commonly observed natural S
A
appearance. The tex-
tures and mesomorphic properties disappeared at 169 °C. When
the isotropic state was cooled to 149 °C, typical bâtonnets
appeared as shown in Fig. 4c; and the fan-shaped texture appeared
again on cooling to 121 °C. The phase transition from the isotropic
*
phase can be distinguished from that of isotropic-N*
to the S
A
phase transition. While for the latter basically spherical nuclei
are observed to grow from the black background of the isotropic
*
melt, the direct transition to a S
A
phase is often accomplished by
the growth of elongated germs (so-called smectic bâtonnets). Sin-
gle bâtonnets show a pronounced anisotropy of their growth kinet-
ics due to the existence of screw dislocations [43].
To confirm the mesophase behavior and to understand the
molecular arrangement of these imidazolium chloride salts in the
crystal and mesophase, power XRD were studied for [Ca-Me-Im]Cl,
[
Ca-Bu-Im]Cl, [Cb-Me-Im]Cl and [Cb-Bu-Im]Cl at different temper-
ature. All these salts displayed layered structures in both the
crystal and liquid crystalline phases, in which the corresponding
layer spacings were determined according to XRD measurements,
as shown in Table 2. For the imidazolium chloride salts, The d spac-
ings of crystalline phase at room temperature (25 °C) exhibited a
little increment with increase of chain length from [Ca-Me-Im]Cl
to [Cb-Bu-Im]Cl, indicating a similar structure for all these salts
in the series.
These imidazolium chloride salts displayed similar XRD results
when they were measured on heating, so we will begin our
discussion with [Cb-Me-Im]Cl. Fig. 6 represents diffractograms
for [Cb-Me-Im]Cl at crystal phase (25 °C) and (b) LC phase
(
95 °C). The crystalline state shows a strong small-angle diffraction
peak corresponding to the layer spacing (29.8 Å), indicating a
highly ordered lamellar structure. A set of diffraction peaks at wide
angle region are observed in Fig. 6a suggesting a crystal structure
section. In consideration of approximately equal spacing between
the crystal layer spacing and the calculated chain length, the
imidazolium chloride salt [Cb-Me-Im]Cl should be organized with
an interdigitated monolayer structure. When it was heated,
Fig. 4. DSC thermograms and optical textures (200Â) of the cholesteryl chloro-
alkylcarboxylic esters: (a) DSC thermograms the first heating/cooling cycle for Ca-
Cl; (b) DSC thermograms the first heating/cooling cycle for Cb-Cl; (c) oily streak
texture of Ca-Cl on heating to 191 °C; (d) focal conic texture of Ca-Cl on cooling to
1
76 °C; (e) oily streak texture of Cb-Cl on heating to 121 °C; (f) focal conic texture of
Cb-Cl on cooling to 110 °C.
[
Cb-Me-Im]Cl exhibited a typical smectic diffractogram with two
reflection peaks (d and d ) and comparison with molecular length
1
2
of a representative compound [Ca-Me-Im]Cl was discussed for
as shown in Fig. 6b, which coincides with the previous report [44].
Almost all wide angle peaks observed in the crystal phase are
absent here, indicating a decrease in the positional ordering. This
observation is consistent with a SmA phase [45]. The sharp
these series materials. For [Ca-Me-Im]Cl, during heating scans, a
*
melting transition (crystalline-to-S
A
phase transition) and an
isotropic phase transition appeared at 127.7 °C and 169.3 °C
Table 1
Phase transitions and enthalpies for imidazolium chloride salts and imidazolium tetrachloroaluminate salts.
Phase transitions (°C)/enthalpy changes (kJ mol )a
À1
Sample
Heating
Cooling
*
[
[
[
[
[
[
[
[
Ca-Me-Im]Cl
Ca-Bu-Im]Cl
Cb-Me-Im]Cl
Cb-Bu-Im]Cl
Ca-Me-Im]AlCl
Cr 127.7/35.1 S*
A
169.3/1.1 I
171.1/1.4 I
127.7/1.3 I
129.6/0.7 I
I 149.0/À1.1 S
I 152.7/À1.3 S
I 113.7/À0.9 S
I 110.3/À0.5 S
121.7/À13.6 Cr
117.7/À19.8 Cr
65.9/À29.4 Cr
64.8/À18.7 Cr
*
*
Cr 131.1/30.0 S
A
A
Cr 77.5/35.6 S*
*
A
A
*
*
Cr 74.9/36.5 S
A
A
Cr 90.1/31.1 N* 162.8/1.0 I
I 140.6/À0.9 N 85.5/À11.8 Cr
*
4
*
*
Ca-Bu-Im]AlCl
Cb-Me-Im]AlCl
Cb-Bu-Im]AlCl
4
Cr 88.5/24.6 N 153.2/0.8 I
I 138.8/À0.4 N 83.4/À13.4 Cr
Cr 74.4/18.7 N* 120.5/1.8 I
Cr 71.3/37.4 N* 113.2/1.8 I
I 116.2/À1.3 N 68.3/À12.4 Cr
*
4
*
4
I 104.3/À1.2 N 65.3/À18.0 Cr
a
Cr, crystalline; N , chiral nematic phase; S*
*
A
, chiral smectic A phase; I, isotropic.

X. Lan et al. / Journal of Molecular Structure 1075 (2014) 515–524
521
the enthalpies for mesophase-isotropic phase transition changed
little. Since the series of imidazolium tetrachloroaluminates
showed similar liquid-crystalline behavior, the discussion was
begun with [Cb-Bu-Im]AlCl
A typical DSC trace of [Cb-Bu-Im]AlCl
Cb-Bu-Im]AlCl showed a melting transition at 71.3 °C and a
N -isotropic phase transition at 113.2 °C on heating cycles. During
4
for these series of materials.
4
is shown in Fig. 7a.
[
4
*
*
*
cooling scans, an isotropic-N transition at 104.3 °C, a N -crystal
transition at 65.3 °C were observed. All the mesophases were iden-
tified by optical textures observed by polarizing optical micros-
copy, and some typical textures are exhibited in Fig. 7. Fan-like
texture (see Fig. 7b) is a natural texture that is often observed in
*
N phase, which can be observed for strongly twisted materials.
Especially the fan-like texture is very similar to that of S*
phase,
A
*
but with a somewhat smoother appearance of the fans. The N
phase of [Cb-Bu-Im]AlCl was recognized by the formation of
spherical nuclei on cooling from the isotropic liquid as shown in
4
*
Fig. 7c, which is different from bâtonnets of S
A
phase observed
for the precursor [Cb-Bu-Im]Cl.
*
The nature of the N phase of [Cb-Bu-Im]AlCl
4
was confirmed by
X-ray diffraction. Fig. 8 displays XRD profiles of crystal phase at
*
room temperature (25 °C) and N phase at mesophase (95 °C) for
[
4
Cb-Bu-Im]AlCl . The crystal state exhibits a strong small-angle dif-
fraction peak corresponding to the layer spacing (39.8 Å) accompa-
nied by a set of diffraction peaks at wide angle regions as shown in
Fig. 7a, indicating a highly ordered lamellar crystal structure. Since
the crystal layer spacing is l < d < 2l, where ‘l’ is the calculated chain
length (29 Å), we propose that for the tetrachloroaluminate salt the
Fig. 5. DSC thermograms and optical textures (200Â) of 1-Cholesteryloxycarbon-
ylmethyl-3-methylimidazolium chloride [Ca-Me-Im]Cl: (a) DSC thermograms the
first heating/cooling cycle; (b) fan-shaped texture on heating to 152 °C; and (c)
bâtonnets from the isotropic melt on cooling to 148 °C.
crystal has an interdigitated bilayer structure, which differs from
*
the precursor [Cb-Bu-Im]Cl. While in the N phase, [Cb-Bu-Im]AlCl
4
exhibited a typical chiral nematic diffractogram with diffusion
bands at both small angle area and wide angle area (d spacing
low-angle peak suggests a lamellar structure with the layer thick-
ness, and the diffuse band at wide angle band (d spacing 5.4 Å)
indicates melted molecular chains with an average intralamellar
distance. There is a little increment of layer thickness from crystal
to mesophase for [Cb-Me-Im]Cl, which can be attributed to a
decrease in the tilting angle of the molecular rod with respect to
the plane normal [46].
4
.9 Å) as shown in Fig. 8b, which coincides with the previous report
[
47]. Compared with the crystal structure, the strong sharp peaks
at small angle became weakness and diffusion should be due to
disappearance of the layer structure when the sample was heated
to mesophase. Especially the wide angle peaks observed in the
crystal phase are became weakness and diffusion here, indicating
a decrease in the positional ordering on heating.
Liquid-crystalline behavior of imidazolium tetrachloroaluminates
The liquid crystalline behavior of the imidazolium tetrachloro-
aluminates were studied by DSC, POM and XRD analysis. The tran-
sition temperatures and enthalpies are reported from the first
heating cycle of the DSC studies and are provided in Table 1, and
the XRD data are shown in Table 2. DSC thermograms show that
these imidazolium tetrachloroaluminates exhibit relatively low
melting point temperatures and relatively low clear point temper-
ature compared with the precursors. In comparison with the imi-
dazolium chloride salts, these imidazolium tetrachloroaluminates
exhibit low enthalpies for crystal-mesophase transition, while
Conductivities of the imidazolium-based LC materials
In order to elucidate the influence of mesophase and the salifi-
cation of the imidazolium-based materials, ionic conductivities of
the synthesized imidazolium chloride salts and the imidazolium
tetrachloroaluminates in mesogenic phase and isotropic phase
were measured as well, which are summarized in Table 2. All these
imidazolium-based materials including the imidazolium chloride
salts and the imidazolium tetrachloroaluminates displayed the
same order of magnitudes of ionic conductivities in mesophase,
Table 2
Interlayer spacing and ionic conductivities for imidazolium chloride salts and imidazolium tetrachloroaluminates in mesogenic phases and other phases.
d-spacing (Å)/temperaturea
Conductivity (Â10À3 S cm )/temperatureb
À1
Sample
Crystal
Mesophase
Mesophase
Isotropic
[
[
[
[
[
[
[
[
Ca-Me-Im]Cl
Ca-Bu-Im]Cl
Cb-Me-Im]Cl
Cb-Bu-Im]Cl
Ca-Me-Im]AlCl
27.5/25 °C
28.3/25 °C
29.8/25 °C
30.2/25 °C
38.0/25 °C
38.4/25 °C
38.2/25 °C
39.8/25 °C
29.3/135 °C
29.7/135 °C
31.6/95 °C
32.2/95 °C
–/95 °C
–/95 °C
–/95 °C
–/95 °C
0.18/25 °C
0.22/25 °C
0.16/25 °C
0.27/25 °C
0.67/25 °C
0.88/25 °C
0.76/25 °C
0.96/25 °C
5.2/135 °C
4.8/135 °C
5.9/95 °C
5.2/95 °C
4
16.1/95 °C
15.9/95 °C
16.3/95 °C
14.6/95 °C
Ca-Bu-Im]AlCl
Cb-Me-Im]AlCl
Cb-Bu-Im]AlCl
4
4
4
a
Determined by XRD measurements in crystal and mesogenic phases at corresponding temperature.
Measured by a RTS-9 four-probe meter.
b

5
22
X. Lan et al. / Journal of Molecular Structure 1075 (2014) 515–524
Fig. 8. XRD profiles of [Cb-Bu-Im]AlCl
and (b) N* phase on heating to 95 °C.
4
at different temperature: (a) crystal at 25 °C,
Fig. 6. XRD diffractograms of 1-cholesteryloxycarbonylpropyl-3-methylimidazo-
lium chloride ([Cb-Me-Im]Cl) at different temperature: (a) crystal phase at 25 °C,
and (b) S*
phase on heating to 95 °C.
A
tetrachloroaluminates presented a higher conductivity increase
both in mesophase and in isotropic phase due to stronger interac-
À
tion among AlCl
4
and imidazolium cations, although very little dif-
ference in conductivity was observed in the imidazolium-based
salts with the same anions. This result is similar to the conductivity
in normal imidazolium-based ionic liquids [49].
Comparative study for imidazolium chloride salts and imidazolium
tetrachloroaluminates
Except for different anions (chloridion and tetrachloroalumi-
nate ion), both the synthesized imidazolium chloride salts and
the imidazolium tetrachloroaluminates contain the same imidazo-
lium cations in the chemical structure. However, there is different
interaction among the imidazolium cations and the anions due to
formation of stronger hydrogen bonds for the latter than that those
in the former, as discussed above in detail. This difference interac-
tion plays a key role in determining the performance difference
between these two kinds of the imidazolium-based salts, which
are mainly reflected in the LC behavior and ionic conductivity men-
tioned above.
This two types of imidazolium-based salts exhibited different
mesophase. All the imidazolium chloride salts displayed S*
phase,
A
*
while the imidazolium tetrachloroaluminates displayed N phase
in both heating and cooling cycles. Furthermore, temperature
range of mesophase is also different between these two types of
ionic LCs. Fig. 9 shows phase transition temperatures of all the imi-
dazolium-based salts on heating cycles. For the imidazolium-based
salts originated from Ca-Cl, the tetrachloroaluminates [Ca-Me-
Fig. 7. DSC thermograms and optical textures (200Â) of [Cb-Bu-Im]AlCl
thermograms of the first heating/cooling cycle; (b) fan-like texture on heating to
1 °C; and (c) spherical nuclei growing from the black background of the isotropic
4
: (a) DSC
9
melt on cooling to 102 °C.
4 4
Im]AlCl and [Ca-Bu-Im]AlCl exhibited much lower Cr-mesophase
transition temperature and wider LC temperature range than the
corresponding chloride salts [Ca-Me-Im]Cl and [Ca-Bu-Im]Cl,
respectively. While for the imidazolium-based salts derived from
Cb-Cl, this variation was not observed. This result indicates that
but the conductivity in isotropic phase was a magnitude greater or
more than that in mesophase due to decrease of viscosity [48]. In
comparison with the imidazolium chloride salts, the imidazolium

X. Lan et al. / Journal of Molecular Structure 1075 (2014) 515–524
523
imidazolium tetrachloroaluminates, the corresponding organiza-
À
tion structures are discussed, as shown in Fig. 10. The Cl -contain-
ing imidazolium salts exhibited layer structure both in crystal and
mesophase states, and should be organized with a ‘head-to-tail’
organization to form interdigitated monolayer structures due to
À
the tight ion pairs (imidazolium cations and tiny Cl ), as shown
À
in Fig. 10a. But The AlCl
4
-containing imidazolium salts displayed
layer structure only in crystal phase, and should be organized in
head-to-head’ arrangements form bilayer structures due to loose
‘
combination of ion pairs in spite of hydrogen-bond and electro-
static attraction interaction, as shown in Fig. 10b. This loose com-
À
bination among imidazolium cations and AlCl
4
was formed rather
À
than tight ion pairs because AlCl
4
ions were large and more space
occupied. When it was heated in this case, this bilayer structure
*
should be destroyed easily to form N phase.
Conclusion
Fig. 9. Phase transition temperatures of all the imidazolium-based salts on heating
cycles. Cr: crystalline phases; S*
: chiral smectic A mesophase; N : chiral nematic
*
A
Two types of imidazolium-based ionic liquid crystals including
imidazolium chloride salts ([Ca-Me-Im]Cl, [Ca-Bu-Im]Cl, [Cb-Me-
Im]Cl and [Cb-Bu-Im]Cl) and imidazolium tetrachloroaluminates
mesophase; Iso: isotropic phase.
À
À
4
the difference between Cl and AlCl
exerts great influence on the
([Ca-Me-Im]AlCl
Im]AlCl ) were synthesized in this work. [Ca-Me-Im]Cl, [Ca-Bu-
Im]Cl, [Cb-Me-Im]Cl and [Cb-Bu-Im]Cl showed chiral smectic A
4 4 4
, [Ca-Bu-Im]AlCl , [Cb-Me-Im]AlCl and [Cb-Bu-
LC behaviors of these imidazolium-based materials.
To give more information about the different molecular
arrangements between the imidazolium chloride salts and the
4
*
Fig. 10. Schematic illustration of the different molecular arrangements in a monolayered (a) for [Cb-Bu-Im]Cl in ‘head-to-tail’ organizing model and bilayered (b) for [Cb-Bu-
Im]AlCl ‘head-to-head’ organizing model in crystal pahse. Where d refers to the layer spacing observed in XRD analysis.
4
1

5
24
X. Lan et al. / Journal of Molecular Structure 1075 (2014) 515–524
confirmed by typical chiral smectic A textures observed by POM
and typical smectic diffractogram measured by XRD. These imi-
dazolium chloride salts contain different cholesteryl component
and different alkyl substituent groups on the imidazole ring. The
samples with the same cholesteryl component showed similar
phase transition temperature and entropy, indicating the choleste-
ryl component influence predominately on the phase transition
rather than alkyl substituents on imidazole ring. The imidazolium
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[
[
[
[
[
[
[
tetrachloroaluminates [Ca-Me-Im]AlCl
4 4
, [Ca-Bu-Im]AlCl , [Cb-Me-
*
Im]AlCl and [Cb-Bu-Im]AlCl displayed chiral nematic (N ) phase
4
4
[
[
[
[
on both heating and cooling cycles, which are different from the
*
corresponding precursor imidazolium chloride salts. The N phase
was testified by POM and XRD studies. DSC thermograms show
that these imidazolium tetrachloroaluminates display relatively
low melting point temperatures and low clear point temperature
compared with the precursors. But the phase transition tempera-
tures and entropies varied little for all these imidazolium tetra-
chloroaluminates. The functional difference in LC behavior and
ionic conductivity were discussed by investigated the structural
[
[
À
À
4
difference between the Cl -containing and AlCl
-containing mate-
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[
[
[
[
rials. The imidazolium chlorides exhibited layer structure both in
crystal and mesophase states, and should be organized with a
‘
head-to-tail’ organization to form interdigitated monolayer struc-
tures due to the tight ion pairs. But the imidazolium tetrachloroa-
luminates displayed layer structure only in crystal phase, and
should be organized in ‘head-to-head’ arrangements form bilayer
structures due to loose combination of ion pairs despite of hydro-
gen-bond and electrostatic attraction interaction.
5048.
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The authors are supported by Project 51273035 supported
by NSFC and the Fundamental Research Funds for the Central
Universities (N130405001).
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